I pulled the ability directly from the Druid, level and all. Honestly, though, I don't think it matters too much... how often does poison come up in the average game, natural or otherwise?
I think transferring the ability from the druid class without any change diminishes the druid class. Class abilities should be unique to the class. Why would the beastman be immune to magical poisons? The druid at least has some affinity to magic while the beastman does not.
Poison usage depends on the PCs and DM. Some people use it a lot, and some very little. I don't think how often it will be used should determine what the ability does.
By the way, Shifting should be an Enhancement not an enchantment.
P.E.A.C.H. Please Evaluate And Critique Honestly. Being nicer and kinder doesn't hurt either. Note I generally only critique 3.5 and Pathfinder material.
Please, please, please when using non-core material, cite to the books. There are too many books to wade through to find the one with the feat, special ability or spell you use.